Martian salts must touch ice to make liquid water, study shows
University of Michigan

In chambers that mimic Mars’ conditions, University of Michigan researchers have shown how small amounts of liquid water could form on the planet despite its below-freezing temperatures.
Liquid water is an essential ingredient for life as we know it. Mars is one of the very few places in the solar system where scientists have seen promising signs of it – in gullies down crater rims, in instrument readings, and in Phoenix spacecraft self portraits that appeared to show wet beads on the lander’s leg several years ago.
No one has directly detected liquid water beyond Earth, though. The U-M experiments are among the first to test theories about how it could exist in a climate as cold as Mars’ climate.
The researchers found that a type of salt present in Martian soil can readily melt ice it touches – just like salts do on Earth’s slippery winter walkways and roads. But this Martian salt cannot, as some scientists suggested, form liquid water by sucking vapor out of the air through a process called deliquescence.